1. Technical Field
The present invention relates to a molded case circuit breaker and more particularly to a current limiting molded case circuit breaker optimized to enable the current limiting circuit breaker to be disposed in a relative small breaker frame size.
2. Description of the Prior Art
Molded case circuit breakers are generally known in the art. An example of such a circuit breaker is disclosed in U.S. Pat. No. 4,891,618. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload or a short circuit or both. An overload normally is about 200-300% of the nominal current rating of the circuit breaker, while a short circuit may be 1000% or more of the nominal current rating of the circuit breaker.
Overload protection is normally provided by a bimetal disposed in series with a load conductor. The bimetal normally consists of two strips of metal having different rates of thermal expansion, bonded together at one end. On a sustained overload, the bimetal will deflect due to the heat and engage the circuit breaker trip bar to trip the circuit breaker.
Short circuit protection may be provided by an electromagnet assembly or by magnetic repulsion forces. Electromagnet assemblies include an electromagnet disposed in series with a load conductor and include a cooperating armature which latches the circuit breaker trip bar during normal conditions. During a short circuit condition, the short circuit current passes through the electromagnet which generates attraction forces to attract the armature and unlatch the trip bar which, in turn, causes the circuit breaker to trip.
Short circuit protection may also be provided by magnetic repulsion members. For example, as disclosed in U.S. Pat. No. 4,891,618, magnetic repulsion members, which consist of flexible shunts are formed in generally a V-shape defining two depending legs. The flexible shunts are used to connect the pivotally mounted contact arlns to the load conductors. During a short circuit condition, the short circuit current flowing in the depending legs of the shunts generate repulsion forces between the depending legs which causes the pivotally mounted contact arms to blow open.
The electromagnet assemblies are normally used to provide short circuit protection where the expected short circuit current is 50,000 amperes or less. Since modern electrical distribution systems are capable of delivering substantially larger short circuit current, for example, 100,000 amperes or more, current limiting molded case circuit breakers are used in such applications. Such current limiting circuit breakers have been known to use magnetic repulsion members to interrupt short circuit currents of 100,000 amperes or more.
Such current limiting circuit breakers are provided in various frame sizes. The frame size refers to a number of important characteristics of the circuit breaker, such as maximum allowable voltage and current, interrupting capacity and physical dimensions of the molded case. For example, U.S. Pat. No. 4,891,618 relates to Westinghouse Series C, R-frame circuit breaker, rated at 600 volts and 1600/2000 amperes.
Since molded case circuit breakers and in particular current limiting molded case circuit breakers are relatively compact, a problem exists to provide current limiting capabilities for a circuit breaker in relatively smaller frame sizes. More specifically, the components in a relatively larger frame size current limiting molded case circuit breaker cannot merely be downsized to provide a current limiting circuit breaker in a smaller frame size.